The human 5-hydroxytryptamine-6 (5HT6) receptor, one of the most recently cloned serotonergic receptors, is a 440-amino acid polypeptide with seven transmembrane spanning domains typical of the G-protein-coupled receptors. It is one of the 14 receptors that mediate the effects of the neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) (Hoyer et al., Neuropharmacology, 1997, 36:419). Within the transmembrane region, the human 5HT6 receptor shows about 30-40% homology to other human 5-HT receptors and is found to be positively coupled to adenylyl cyclase.
The prominent localization of 5HT6 receptor mRNA in the nucleus accumbens, striatum, olfactory tubercle, substantia nigra, and hippocampus of the brain (Ward et al., Neuroscience, 1995, 64:1105) together with its high affinity for several therapeutically important antipsychotics and antidepressants, suggest a possible role for this receptor in the treatment of schizophrenia and depression. In fact, the prototypic atypical antipsychotic agent clozapine exhibits greater affinity for the 5HT6 receptor than for any other receptor subtype (Monsma et al., J. Pharmacol. Exp. Ther., 1994, 268:1403).
Although the 5HT6 receptor has a distinct pharmacological profile, in vivo investigation of receptor function has been hindered by the lack of selective agonists and antagonists. Recent experiments demonstrated that chronic intracerebroventricular treatment with an antisense oligonucleotide, directed at 5HT6 receptor mRNA, elicited a behavioral syndrome in rats consisting of yawning, stretching, and chewing. This syndrome in the antisense-treated rats was dose-dependently antagonized by atropine (a muscarinic antagonist), implicating 5HT6 receptor in the control of cholinergic neurotransmission. Therefore, 5HT6 receptor antagonists may be useful for the treatment of memory dysfunction (Bourson et al., J. Pharmacol. Exp. Ther., 1995, 274:173), and to treat other central nervous system (CNS) disorders.
The high affinity of a number of antipsychotic agents for the 5-HT6 receptor, in addition to its mRNA localization in striatum, olfactory tubercle and nucleus accumbens suggests that some of the clinical actions of these compounds may be mediated through this receptor. Compounds which interact with, stimulate, or inhibit the 5-HT6 receptor are commonly referred to as 5-HT6 ligands. In particular, 5-HT6 selective ligands have been identified as potentially useful in the treatment of certain CNS disorders such as Parkinson's disease, Huntington's disease, anxiety, depression, manic depression, psychoses, epilepsy, obsessive compulsive disorders, migraine, Alzheimer's disease (enhancement of cognitive memory), sleep disorders, feeding disorders such as anorexia and bulimia, panic attacks, attention deficit hyperactivity disorder (ADHD), attention deficit disorder (ADD), withdrawal from drug abuse such as cocaine, ethanol, nicotine and benzodiazepines, schizophrenia, bipolar disorder, and also disorders associated with spinal trauma and/or head injury such as hydrocephalus. Such compounds are also expected to be of use in the treatment of certain gastrointestinal (GI) disorders such as functional bowel disorder and irritable bowel syndrome (See for ex. B. L. Roth et al., J. Pharmacol. Exp. Ther., 1994, 268, pages 1403-14120, D. R. Sibley et al., Mol. Pharmacol., 1993, 43, 320-327, A. J. Sleight et al., Neurotransmission, 1995, 11, 1-5, and A. J. Sleight et al. Serotonin ID Research Alert, 1997, 2 (3), 115-8). Furthermore, the effect of 5-HT6 antagonist and 5-HT6 antisense oligonucleotides to reduce food intake in rats has been reported (Br. J. Pharmac., 1999 Suppl. 126, page 66 and J. Psychopharmacol Suppl. A64, 1997, page 255).
Therefore, it is an object of this invention to provide compounds which are useful as therapeutic agents in the treatment of a variety of central nervous system disorders related to or affected by the 5-HT6 receptor.
It is another object of this invention to provide therapeutic methods and pharmaceutical compositions useful for the treatment of central nervous system disorders related to or affected by the 5-HT6 receptor.
The following patents and publications also provide relevant background to the present invention. All references cited below are incorporated herein by reference in their entirety and to the same extent as if each reference was individually incorporated by reference. U.S. Pat. Nos. 6,100,291, 6,133,287, 6,191,141, 6,251,893, 6,686,374, 6,767,912, 6,897,215, 6,903,112, and 6,916,818; Published U.S. Application Nos. 2005/0124603, and 2005/0171118.